1. Field of the Invention
The present invention relates to membrane electrode assembly structures for electrochemical fuel cells and more particularly to modifications to improve tolerance to voltage reversals.
2. Description of the Related Art
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of economically delivering power with environmental and other benefits. To be commercially viable however, fuel cell systems need to exhibit adequate reliability in operation, even when the fuel cells are subjected to conditions outside the preferred operating range.
Fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include polymer electrolyte membrane (PEM) fuel cells that comprise a polymer membrane as electrolyte and operate at relatively low temperatures.
A broad range of reactants can be used in PEM fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant can be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a PEM fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
Polymer electrolyte membrane (PEM) fuel cells employ a membrane electrode assembly (MEA) which comprises an ion-exchange membrane disposed between the two electrodes. Separator plates, or flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA.
Each electrode contains a catalyst layer, comprising an appropriate catalyst or an admixture of appropriate catalysts, which is located next to the ion-exchange membrane. The catalyst may be a metal black, an alloy, an unsupported or supported metal catalyst. A commonly used catalyst is, for example, platinum supported on carbon. The catalyst layer typically contains ionomer, which may be similar to that used for the ion-exchange membrane (for example, up to 30% by weight Nafion® brand perfluorosulfonic-based ionomer). The catalyst layer may also contain a binder such as polytetrafluoroethylene.
The electrodes may also contain a substrate (typically a porous electrically conductive sheet material) that may be employed for purposes of reactant distribution and/or mechanical support. Optionally, the electrodes may also contain a sublayer (typically containing an electrically conductive particulate material, for example, finely comminuted carbon particles, also known as carbon black) between the catalyst layer and the substrate. A sublayer may be used to modify certain properties of the electrode (for example, interface resistance between the catalyst layer and the substrate).
Electrocatalyst can be incorporated at the electrode/membrane interface in polymer electrolyte fuel cells by applying it as a layer on either an electrode substrate or on the membrane itself. In the former case, electrocatalyst particles are typically mixed with a liquid to form a slurry or ink which is then applied to the electrode substrate. While the slurry preferably wets the substrate surface to a certain extent, the slurry may penetrate into the substrate such that it is no longer catalytically useful since the reaction zone is generally only close to the ion-exchange membrane. Comparatively lower catalyst loadings can thus typically be achieved by coating the ion-exchange membrane with a catalyst layer while still maintaining performance. In addition to waste of catalyst material, a thicker electrocatalyst layer as typically coated on electrode substrates may also lead to increased mass transport losses.
Typical methods of preparing a catalyst coated membrane (CCM) also start with the preparation of a slurry. A slurry typically comprises a carbon-supported catalyst, the polymer matrix/binder and a suitable liquid vehicle such as, for example water, methanol or isopropanol. The slurry is then either directly applied onto the membrane by, for example screen printing, or applied onto a separate carrier or release film from which, after drying, it is subsequently transferred onto the membrane using heat and pressure in a decal process. Alternatively, the CCM may be made by other known methods such as vapor deposition, casting or extrusion.
Efficiency of the MEA in the fuel cell is typically affected by the quality of the contact between the catalyst layer and the ion-exchange membrane. When the quality of such a contact is relatively poor, partial or complete delamination of the MEA may result over time. CCMs typically have a better contact between catalyst layer and ion-exchange membrane as compared with GDEs bonded to an ion-exchange membrane, particularly with low catalyst loadings such as, for example, less than 0.3 mg/cm2 of platinum catalyst. It may be difficult to prepare a suitable GDE with such low catalyst loadings.
However, there may also be indirect costs associated with coating catalyst layers on ion-exchange membranes. Both the catalyst and the ion-exchange membrane are relatively expensive components found in a typical PEM fuel cell, particularly as compared to gas diffusion layers. Errors in coating a catalyst layer on an ion-exchange membrane may result in the entire CCM being rejected.
While CCM techniques typically result in an interface with higher connectivity or contiguity between the catalyst and the ion-exchange membrane and thus better performance in the corresponding fuel cell, improvements are still needed in over-all fuel cell performance and durability.